The present invention relates generally to a method of digital communication and to a digital communication system.
The applicant""s known vehicle tracking system, which is in use in the United Kingdom, allows the police to track and recover stolen vehicles. In this known system, a small transceiver unit is hidden in a vehicle. The unit is activated by an activation signal to transmit signals when the vehicle is stolen. The activation signal is generated from a network of base stations. Police vehicles are fitted with receivers, which enable them to receive the signals transmitted by the transceiver unit in the stolen vehicle. In this manner, the police can track, locate and recover the vehicle.
Additional and more sophisticated functions could be incorporated in the tracking system if there were a two-way link between the transceiver unit in each vehicle and a base station, usually the nearest base station, with that base station acting additionally as a receiver for receiving data transmitted in signals from the transceiver unit. Such two-way or duplex communication is not possible with the known system because of the disparity in the power available from an on-board transceiver unit and from a base station. In particular, the base stations have transmitters having a power of 25 watts and a range of the order of 30 miles (about 50 km). The on-board transceiver units each have a transmitter whose power is 1 watt and with a maximum range of 1.5 miles (about 2.5 km), which is often not sufficient to reach a base station. It is not possible simply to increase the power of the on-board transmitter units as this would increase their size and their cost, and this is not acceptable to the market.
It is possible to reduce the bandwidth of the transmissions to extend the range of transmissions from the on-board transceiver units. This would have the effect of reducing the speed at which the information is transmitted, but for many applications, such as in a tracking system, such a speed reduction is acceptable. Normally, the use of a reduced bandwidth transmitter unit would increase the cost of each on-board transceiver unit because high accuracy components in the transceiver unit would be required. This is because the uncertainty or error in the transmit carrier frequency must normally be less than the bandwidth of the signal being transmitted in order for the receiver to be able to locate the data. However, the market will not accept units having a significantly higher cost and therefore high accuracy components cannot be used in such a transceiver.
The present invention seeks to increase the range of a transmitter without increasing its size or cost. The present invention further seeks to increase the range of transmission of signals containing data from a transmitter without increasing its size or cost. The present invention has particular application to a vehicle tracking system but can be used in many applications, especially where low cost transmitters are used or required.
In EP-A-0583522 there is disclosed a remote position determination system. The spatial position of a plurality of mobile transceivers can be determined by a base station which receives positioning signals from the mobile transceivers. The mobile transceivers use a spread spectrum frequency-hopped transmission mode, with each mobile transceiver having its own characteristic frequency hopping pattern in order for the base station to be able to identify which mobile transceiver is transmitting on the basis of the particular frequency hopping pattern in the signal received. As the frequency of transmission used by each mobile transceiver is therefore critical to it being properly identified by the base station, it is essential that the base station transmits timing synchronising signals to the mobile transceivers and the mobile transmitters accordingly have to be relatively complex (and therefore expensive) in that they must contain memories and circuits so that the correct frequency and pattern of frequency hopping is transmitted by each mobile transceiver. Furthermore, the signals transmitted by the mobile transceivers are not modulated with and therefore do not contain any data to be transmitted to the base station because the only information used by the base station is the fact that a particular identified mobile transceiver is actually transmitting. There is therefore no two-way communication of data between the base station and the mobile transceivers.
In EP-A-0550146, there is disclosed a digital processor for processing multiple analog signals respectively received from multiple mobile transmitters.
According to a first aspect of the present invention there is provided a method of transmitting and receiving data, the method comprising the steps of: transmitting a narrow band data signal at an unknown frequency within a known range of frequencies; receiving the data signal within the known range of frequencies; dividing at a receiver the range of frequencies into plural frequency bands each of width less than the uncertainty in the transmission frequency of the data signal; detecting the presence of said data signal in at least one of said frequency bands; and, demodulating the detected data signal.
The demodulating step preferably includes the step of centring at least one narrow band filter on the detected data signal frequency bands.
The frequency of the or each narrow band filter in the demodulation step is preferably determined in the detection step.
The time of data signals in the narrow frequency bands is preferably determined in the detection step and used in the demodulation step.
In a most preferred embodiment, the step of detecting the presence of data signals in at least one of the frequency bands comprises the step of detecting the presence of a data signal in plural frequency bands, the amplitude of the data signal in each band being compared with a normalised value to determine a quality value of the data signal in each band.
The transmitted data signal preferably includes a flag sequence and a xe2x80x9ccentre-of-gravityxe2x80x9d calculation is preferably performed on the quality values to provide a central frequency and a central time for each flag sequence, the results of the centre-of-gravity calculation being utilised during demodulation of the data signal.
The frequency of the transmitted data signal may be varied between successive transmissions. This ensures that a good signal will be received even if there is line interference at a particular frequency as the subsequent transmission will be at a frequency which is unlikely to be affected by the same interference.
The data signal is preferably transmitted as two sub-channels, the detection and demodulation steps being carried out on each sub-channel, the respective demodulated data signals being weighted and summed according to the quality of the signals determined in the detection steps.
The received signal can include plural narrow band data signals each occupying a distinct portion of the channel, the receiver receiving said narrow band data signals substantially concurrently. This allows the receiver to receive transmissions from several transceiver units substantially concurrently.
Plural demodulation steps may be carried out following each detection step.
An embodiment of the present invention increases the effective range of the transmitter by providing, in effect, narrow band communication between the transmitter and the receiver. This increases the range for the same power, but at some decrease in the rate of data transmission which can be used. As this narrow band communication is provided by the signal processing techniques at the receiver, it is not necessary to provide a more accurate, and therefore more costly, narrow band transmitter.
In an embodiment, the data signals have been modulated at a transmitter by the use of direct FSK (frequency shift keying) modulation.
A data signal preferably comprises a data packet following a flag sequence of bits and the detection step detects the existence of the flag sequence. In a presently preferred embodiment, data signals in individual narrow frequency bands are compared with a wanted flag signal at regular intervals. Accurate timing, frequency and quality information is obtained from these comparisons, allowing the detection step to be optimised.
Preferably, for security of detection, the data packet is transmitted between leading and trailing flag sequences, and the detection step requires the detection of both flag sequences.
Preferably, it is required that both the leading and trailing flag sequences are detected to verify the presence of a data packet. This greatly reduces the possibility of false detections. Furthermore, the frequency and timing information for the demodulation step is determined from the flag sequences. The presence of a flag sequence before and after each data packet enables the frequency and timing to be used during demodulation to be re-measured at regular intervals.
The data signal may have plural data packets separated from each other by flag sequences. Thus, where there are n data packets, there would be n+1 flag sequences in the data signal. The use of flag sequences to provide a means of detecting the presence of data, and to provide frequency and timing information to enable its demodulation, means that external synchronisation between the transmitter unit and a receiver is not required.
In a preferred embodiment, the step of detecting the presence of data signals in individual narrow frequency bands comprises the step of detecting the presence of a flag sequence in plural narrow frequency bands, the amplitude of the flag sequence bits in each band being compared with a normalised value to determine a quality value of the flag sequence bits in each band, the quality values being plotted against frequency and time.
Preferably, a xe2x80x9ccentre-of-gravityxe2x80x9d calculation is performed on the quality values to provide a central frequency and a central time for each flag sequence, the results of the centre-of-gravity calculation being utilised during demodulation of the data signal.
According to a second aspect of the present invention, there is provided apparatus for transmitting and receiving and demodulating transmissions, the apparatus comprising: a transmitter for transmitting a narrow band data signal at an unknown frequency within a known range of frequencies; and, a receiver for receiving the data signal within the known range of frequencies, the receiver having means for dividing the range of frequencies into plural frequency bands each of width less than the uncertainty in the transmission frequency of the data signal; means for detecting the presence of said data signal in at least one of said frequency bands; and, means for demodulating the detected data signal.
According to a third aspect of the present invention there is provided a method of receiving data transmitted as a narrow band data signal at an unknown frequency within a known range of frequencies, the method comprising the steps of: receiving the data signal within the known range of frequencies; dividing at a receiver the range of frequencies into plural frequency bands each of width less than the uncertainty in the transmission frequency of the data signal; detecting the presence of said data signal in at least one of said frequency bands; and, demodulating the detected data signal.
According to a fourth aspect of the present invention there is provided a receiver for receiving and demodulating a data signal transmitted at an unknown frequency within a known range of frequencies, the receiver comprising: means for dividing the range of frequencies into plural frequency bands each of width less than the uncertainty in the transmission frequency of the data signal; means for detecting the presence of said data signal in at least one of said frequency bands; and, means for demodulating the detected data signal.
In a preferred embodiment, said receiver comprises an analogue to digital converter (ADC), and said dividing means comprises a Fast Fourier Transform (FFT).
In an embodiment, the demodulation means comprises filtering means arranged to pass only a central frequency determined by said detecting means.
Preferably, said division, said detection, and said demodulation are performed digitally in a digital signal processor.
The receiver preferably has detection and demodulation means for detecting and demodulating data signals transmitted in two sub-channels, and further preferably comprises adding means for weighting and summing the respective demodulated data signals according to the quality of the signals determined in the detection steps.
By enabling true two-way data communication between a base station (which may be relatively high power and expensive) and a plurality of transceivers (which may be low power and cost), the present invention greatly enhances the value and usefulness of a digital communications system. For example, when a transceiver has been activated by a base station, the transceiver can send a data signal to the base station to acknowledge that it has been activated, thus providing assurance to the base station that the activation signal has been received and saving the base station from having to repeat the signal many times to ensure that it had been received as in the past. A de-activation signal may be sent to the transceiver which can also be acknowledged by the transceiver so that the base station can be confident that the de-activation signal has been received and thus preventing false or erroneous activations from being maintained. Data concerning the status of the transceiver (such as the status of the transceiver unit, its battery voltage, etc.) can be sent to the base station which can therefore continually monitor the status of the transceiver unit. From time to time, the base station can send test signals to each of the transceiver units to ensure that the units are working properly, with the transceiver units being able to respond appropriately; in the prior art vehicle tracking system described above, for example, the only way of testing a transceiver unit was by bringing the vehicle in on a regular basis for servicing and testing each unit by making physical connections to the unit during each service.
The present invention has particular applicability to a vehicle tracking system. Thus, there may be provided a vehicle tracking system having a system or receiver according to any of the aspects described above. There may also be provided a method of transmitting and receiving data or a method of receiving data as described above in a vehicle tracking system.